Outbreaks of organic mercury poisoning in the United States, Japan and Iraq have had particularly severe consequences for fetal and neonatal victims including severe mental and motor retardation. The pregnant monkey (Macaca mulatta or speciosa) is being employed as a model for studies on minimal toxic levels of methylmercury, extent and rate of transplacental passage, movement into breast milk, and the sites of uptake of organic mercury in body tissues, especially the brain. Completed studies on placental transport suggest that methylmercury203chloride crosses more readily from mother to fetus than the reverse. Consequently, at birth, fetal levels of Hg203 are higher than are maternal ones after repeated maternal ingestion of the compound. The half-life of Hg203 in the adult macaque is approximately 55 days. Infants of lactating mothers ingesting methylmercury203chloride over many weeks achieve only about 1/100th of the maternal blood levels of Hg203 suggesting only limited transfer during nursing. Chronic exposure to methylmercury has demonstrated that a weekly dose of 2.5 to 5.0 milligrams/hg is invariably fatal for the pregnant macaque and her fetus. Lower dosages are being employed to ascertain the minimal levels at which behavioral and neuroanatomical damage can be observed. Maternal blood, cord blood, hair, placental and breast milk samples derived from human pregnancies are being analyzed by selective atomic absorption techniques for organic and inorganic mercury levels. This data will provide information on baseline levels of mercury in a "typical" Midwestern population of mothers and their infants.